In a large bowl, cream together butter and sugar until light. Beat in egg and vanilla. In a small bowl, whisk together flour, graham cracker crumbs, baking powder and salt. Add to butter mixture and mix at a low speed until combined.

Divide dough in half and press half of dough into an even layer on the bottom of the prepared pan. Place chocolate bars over dough. 2 king-sized Hershey’s bars should fit perfectly side by side, but break the chocolate (if necessary) to get it to fit in a single layer no more than 1/4 inch thick. Spread chocolate with marshmallow creme or fluff. Place remaining dough in a single layer on top of the fluff (most easily achieved by flattening the dough into small shingles and laying them together).